1. Field of the Invention
The present invention relates to a GPS receiving system and, more particularly, to a GPS receiving system capable of determining relative positions and relative velocities of moving objects such as spacecraft, aircraft and vehicles with high accuracy.
2. Description of the Related Art
GPS (Global Positioning System) is known as a radio navigation system for obtaining the three-dimensional location by interpreting a positioning radio wave (GPS signal) transmitted from a plurality of artificial satellites (GPS satellites) that orbit the earth.
Presently twenty-seven GPS satellites (not counting spares) are orbiting and transmitting GPS signals. The GPS determines distances between a GPS receiver and at least four viewable GPS satellites in order to perform reliable positioning. A distance between a GPS receiver and a GPS satellite is obtained from the difference between a receiver clock and a satellite clock multiplied by the radio wave propagation velocity, and the distance thus obtained is referred to as a pseudorange.
Moving objects such as spacecraft, aircraft and vehicles carrying a GPS receiver obtain information concerning positions and velocities of themselves (GPS absolute navigation). On the other hand, the use of GPS between moving objects carrying their own GPS receivers makes it possible to determine the relative position and the relative velocity of a moving object with respect to another moving object (GPS relative navigation).
The GPS relative navigation improves positioning accuracy by offsetting errors common to both of the GPS receiver. For example, the relative positions and relative velocities are more accurately determined by receiving GPS signals transmitted simultaneously from a GPS satellite common to each of the GPS receivers. Therefore, efforts have been made to develop techniques for receiving GPS signals from a common GPS satellite and for receiving GPS signals at an equivalently identical time.
The number of GPS satellites to be traced with one GPS receiver has recently become greater than 12, which is sufficiently larger than the number (4) requisite for reliable positioning. Providing a sufficiently wide overlapping area in the view fields of GPS antennas between moving objects in relative navigation has made it easier to obtain the requisite number of GPS signals transmitted from common GPS satellites.
Commonizing the observed time on the basis of software or using hardware for obtaining an external synchronized signal are techniques known for receiving GPS signals at an equivalently identical time.
A description will now be made on a conventional GPS system with reference to the GPS receivers disclosed in Japanese Patent Laid-Open No. 2743/1998. In the block diagram of GPS receivers shown in FIG. 13, the preceding and following vehicles are two moving objects which are in relative navigation along a road in the same direction.
The preceding and following vehicles have, respectively, relative distance measuring devices S1 and S2, GPS antennas 11 and 21, GPS receiver units 12 and 22, controllers 13 and 23, communication units 14 and 24 for communication between the vehicles and ground antennas 15 and 25.
The controllers 13 and 23 include position calculation units 131 and 231 and follow up control units 132 and 232 for controlling the operation of the transmissions and brakes (not shown), etc.
The preceding and following vehicles take GPS signals into the GPS receiver units 12 and 22 through the GPS antennas 11 and 21, respectively. Since GPS signals are transmitted from each GPS satellite at a constant interval (period: T), time information embedded in the GPS signals is utilized as a synchronized signal.
Relative navigation between the preceding and following vehicles will now be described with reference to the communication timing chart shown in FIG. 14.
The preceding vehicle determines its present position Fn based on a GPS signal fGPS (n) received at a time t=n. Then the GPS receiver unit 12 of the preceding vehicle determines its own velocity and moving direction from the present and previous positions, and the position calculation unit 131 in the controller 13 estimates a position F*n+1 of t=n+1 and transmits it to the following vehicle at a time an.
The position calculation unit 231 in the controller 23 of the following vehicle determines its present position Rn+1 based on a GPS signal fGPS (n+1) received at a time t=n+1, and compares it with the estimated position F*n+1 of the preceding vehicle taken into via the communication units 14 and 24 in advance to determine a relative position of t=n+1 of the two vehicles at a time bn+1.
Performing this process periodically makes it possible to always measure the relative position of the following vehicle with respect to the preceding vehicle.
This method is based on an assumption that the GPS receivers are accurately synchronized with the time information embedded in the GPS signals and has an advantage of removing the need for an external synchronized signal. Inaccuracy in synchronization will lose the simultaneity and lead to a serious error in determination. Inaccuracy in synchronization occurs on a receiver clock under the influence of temperature etc.
As described above, synchronization of a receiver clock with the time information embedded in the GPS signals is very important for performing GPS relative navigation accurately. It should be, pointed out that synchronization utilizing an external synchronized signal requires separate hardware and makes a system costly. It should be also pointed out that equivalent synchronization utilizing conventional software requires receiver clocks to be in highly accurate synchronism (on the order of 10 xcexcs, for example) with time information embedded in GPS signals.
The invention has its object to provide a GPS receiving system which achieves an equivalent synchronization in time measurement of GPS signals utilizing software to determine relative positions and relative velocities of moving objects with high accuracy.
According to the invention there is provided a GPS receiving system which performs a relative navigation process after correcting a pseudorange with a time tag error attributable to inaccuracy of receiver clocks of first and second moving objects in GPS relative navigation. The time tag error is preferably calculated from range rates and clock biases of the first and second moving objects. A differential computation unit calculates a difference between the first pseudorange and the second pseudorange commonized by a time tag commonizing unit. The correction in a time tag error correction unit is performed for the selected GPS data with a common GPS satellite identification number.
According to the invention there is also provided a GPS receiving system which performs a relative navigation process after correcting absolute error of a time tag with a clock bias in a time tag error correction unit. The correction in a time tag error correction unit is performed for the selected GPS data with a common GPS satellite identification number. A differential computation unit calculates a difference between the first pseudorange and the second pseudorange commonized by a time tag commonizing unit.